Port the core Repeater class (push/stop executor, async iterator protocol,
overflow protection) from @repeaterjs/repeater into src/repeater.ts, stripping
buffers and combinators we don't use. This removes the 7-year-old dependency
while keeping the same API surface for all current and future adapters
(Redis, WebSocket, SSE, Iroh).
Also add the async utility research doc for reference.
